1. Field of the Invention
The present invention relates to an image fixing device and a method thereof used in a machine employing the electrophotographic method such as a copying machine, facsimile apparatus or printer.
2. Discussion of the Related Art
In a copying machine, for example, which utilizes electrophotographic method, it is necessary to fix an unfixed toner image transferred on a recording sheet to form a permanent image, and therefore, fixing methods such as the vapor fixing, cold pressure fixing and heat fusing have been conventionally used for permanent image formation. However, when the vapor fixing method is employed, a solvent vaporizes and diffuses, which causes an unpleasant odor. The cold pressure fixing method is inferior to other methods in fixing capability, and besides, there is an economic problem such that the cold pressure fixing requires to use expensive pressure sensitive toner, for example, capsule toner. So, both cold pressure fixing and vapor fixing are not used so widely at present.
Accordingly, heat fusing method, which melts the toner by heating and fuses it to the surface of the recording sheet, is widely employed in fixing an unfixed toner image. There are various types of heat fusing device which bring the heat fusing method into practice, and in particular, the device employing a heat roller method is generally used. As shown in FIG. 1, the device of such type comprises a heat roller 1 and a pressure roller (pressure applying means) 2. The heat roller 1 comprises a metal cylindrical core 3, a heater 4 such as an infrared ray lamp provided inside of the metal cylindrical core 3 and a releasing layer 5 covering the peripheral surface of the metal cylindrical core 3. The releasing layer 5 is formed on purpose to prevent attachment of toner, which has transferred from the surface of the recording sheet, to the peripheral surface of the core 3 and is made of heat-resisting material such as fluororesin, silicone rubber or silicone resin. For example, in a small-sized copying machine, the core 3 is a cylinder made of material such as aluminum, aluminum alloy, copper or copper alloy, and the diameter of which is 20 mm to 40 mm, thickness is 0.5 mm to 3 mm. The core 3 is coated with fluororesin as a releasing layer 5 whose thickness is 20 .mu.m to 100 .mu.m.
The pressure roller 2 is disposed to press-contact the heat roller 1 and apply pressure thereto, and comprises a core 6 and a releasing layer 7 made of heat-resisting rubber and coating the peripheral surface of the core 6 to improve paper stripping capability. As the heat-resisting rubber, for example, silicone rubber or fluororubber can be used. By rotating the heat roller 1 and pressure roller 2, a recording sheet 9 on which an unfixed toner image 8 is formed is passed through a nip area between the heat roller 1 and pressure roller 2, where the heat and pressure are applied to the recording sheet 9 to fix the toner image 8.
In comparison with other heat fusing methods, such as radiant fusing or oven fusing, the heat efficiency of the heat roller method is higher; consequently, a smaller amount of electric power is consumed and high-speed fixing can be performed. Even if the recording sheets are jammed, temperature can be controlled with ease so that the temperature of the recording sheet is not extremely higher than that of the heat roller. Accordingly, since there is little possibility of causing a fire, the heat roller method is moreover advantageous and is most widely used at present.
The fixing device employing the heat roller method has a problem that wrinkling is apt to be generated on the recording sheet 9 when the sheet passes through a nip between the roller 1 and roller 2. To resolve the problem, a heat roller 1 with a gradient on its peripheral surface, for example, a heat roller 1 with the diameter of the central portion smaller than that of the end portions by 30 .mu.m to 200 .mu.m, approximately (hereinafter referred to this heat roller as a reversed crown roller), has been conventionally used. Use of the reversed crown roller generates difference in carrying speed of the recording sheet in the direction of width of the recording sheet, whereby the recording sheet is pulled in the direction of both ends and generation of wrinkling can be prevented.
Generally, to manufacture the reversed crown roller, first the core 3 is plastically deformed by extruding a bar or pipe, and is cut in plural rollers having a predetermined length. Then the peripheral surface of the core 3 is formed in a shape of reversed crown by cutting work by lathing or by centerless polishing, and is coated with the releasing layer 5. However, there has been a problem that accuracy is required in forming the core 3 in the shape of reversed crown so that the generation of wrinkling on the recording sheet 9 is prevented; therefore, the cost increases. To reduce the cost of manufacturing the heat roller 1, inventors of the present invention has studied to prevent occurrence of wrinkling on the recording sheet 9 while using a roller having a uniform diameter over the longitudinal direction.
In carrying out the cutting work on the core 3 to form into the shape of the reversed crown, accuracy is required, and as a result the peripheral surface of the core 3 is finished to be almost a mirror surface. Therefore, despite the releasing layer 5 formed on the peripheral surface of the core 3, there is a fear that the releasing layer is debonded from the core 3 because the adhesive force between the core 3 and the releasing layer 5 is weakened though the life of the releasing layer still remains. Conventionally, to prevent debonding of the releasing layer, the peripheral surface of the core 3 is roughened, for example, by sand blasting on purpose to increase the mechanical coupling force (anchor effect) by increase of area which contacts the releasing layer.
If the roller having uniform diameter over the longitudinal direction for which the inventors are seeking is realized, the core 3 having uniform diameter over the longitudinal direction which is made by extruding alone may be used; therefore polishing for forming the core 3 into the shape of reversed crown is unnecessary. However, since the surface of the core 3 after extruding is also like a mirror surface, the adhesive force between the core 3 and the releasing layer 5 should be improved by the surface roughening process which is complicated and expensive to prevent alebonding of the releasing layer 5 from the core 3 before expiration of life of the releasing layer 5.
As another factor causing debonding of the releasing layer 5 from the core 3, not only roughness of the surface of the core 3, but also the influence of the pressure applied to the nip can be considered. That is, repeat of application of the pressure to the nip makes the releasing layer 5 wearing. Besides, since there is a fear that coating the core 3 with the releasing layer 5 is insufficient to prevent fusing of toner 8 from the recording sheet 9 to the heat roller 1, the releasing agent is supplied to the surface of the releasing layer 5. Silicone oil, mainly used as the releasing agent, infiltrates into pinholes, slight scratches or cracks, thus gradually reducing the adhesive force between the releasing layer 5 and the core 3. Moreover, a stripper finger has been conventionally used for stripping the recording sheet 9 from the heat roller 1 compulsorily. Such stripper finger always scratches the releasing layer 5 with its sharp point, whereby generation of the scratches or cracks and wear of the releasing layer 5 is accelerated, and accordingly the deterioration of the releasing layer 5 is induced. This wear or deterioration has been the cause of debonding of the releasing layer 5 from the core 3.